1. Field of the Invention
This application is the U.S. National Stage of PCT/JP2010/066517 filed Sep. 24, 2010, claiming priority of Japan Patent App. No. 2009-221404 filed Sep. 25, 2009, and the contents of said PCT application and said priority application are hereby incorporated by reference herein, in their entirety. The present invention relates to an optical waveguide element module, and more particularly, to an optical waveguide element module including a connector where an external signal line for inputting a modulation signal into a modulation electrode of the optical waveguide element is connected, a relay substrate for connecting modulation electrodes of the optical waveguide element and the connector, and the relay substrate having a relay line and a filter circuit on the relay substrate.
2. Description of Related Art
In the fields of optical communication or optical measurement of the related art, an optical waveguide element, where an optical waveguide is formed on a substrate, such as an optical modulator, is widely used as means for controlling the optical wave.
Such an optical waveguide element is provided with a modulation electrode for modulating the optical wave propagating through the optical waveguide, and the modulation signal is input to the modulation electrode using the connector connected to the external signal line. For this reason, in order to efficiently input the modulation signal to the modulation electrode from the external signal line, it is necessary to achieve an impedance matching between the external signal line and the modulation electrode and prevent reflection of the modulation signal in the transmission line.
FIG. 1 is a diagram illustrating an example of the optical waveguide element module. The optical waveguide element includes an optical waveguide 2 formed on the substrate 1 made of a material having an electro-optic effect or the like, a modulation electrode 3 for modulating the optical wave propagating through the optical waveguide 2 (as to the modulation electrodes, there are a signal electrode and a ground electrode; but, FIG. 1 only illustrates an arrangement of the signal electrode for simplicity), and the like. An input optical fiber 4 for inputting the optical wave and an output optical fiber 5 for outputting the modulated optical wave are connected to the optical waveguide element. The optical waveguide element is housed in a hermetically sealed state within the casing 9 to constitute an optical waveguide element module. The modulation signal from a driver 6 provided in the outer side of the optical waveguide element module is applied via the connector 8 to the modulation electrode 3 of the optical waveguide element.
The impedance from the driver 6 to the connector 8 is typically set to 50Ω. If the impedance of the modulation electrode of the optical waveguide element is 40Ω, reflection of the modulation signal occurs between the connector 8 and the modulation electrode 3 due to impedance mismatching of the transmission line as described above. In order to address such problems, the relay substrate 7 is provided between the connector 8 and the optical waveguide element, and a resistor 11 (in this case, 10Ω resistor) is arranged in the relay substrate 7 as shown in FIG. 2, so that the apparent impedance in the optical waveguide element side as seen from the connector 8 is set to 50Ω.
Although such a method of adjusting the impedance provides a certain effect for reflection suppression of the microwave which is a modulation signal, there is a problem in that the microwave is attenuated by the resistor 11, and it is difficult to effectively apply the modulation signal to the optical waveguide element.
Meanwhile, in recent years, in order to evaluate the optical waveguide element, a so-called jitter characteristic indicating a temporal fluctuation of the optical signal obtained when the optical waveguide element is driven has attracted attention. The jitter is an index indicating a temporal fluctuation of the optical signal and is defined as a width of crossing points of the signal obtained by integrating the optical eye-pattern waveforms.
In order to improve the jitter of the optical signal obtained by driving the optical waveguide element, it is necessary to improve the characteristics of the optical waveguide element or a driver for driving and controlling the optical waveguide element as follows.
(1) Driver
In order to amplify the input electric signal without deterioration, the gain is set to have a flat frequency characteristic from a low frequency range to a high frequency range.
(2) Optical Waveguide Element
In order to convert the input electric signal into the optical signal without deterioration, the frequency of the electric/optical conversion response is set to have a flat frequency characteristic from a low frequency range to a high frequency range.
Although the jitter described above does not occur if the frequency characteristic in the driver and the optical waveguide element is infinitely flat (no frequency dependence) as described above, in practice, neither the driver nor the optical modulator have flat frequency characteristics in a low frequency range, and the jitter occurs since a frequency characteristic in the high frequency range tends to deteriorate under the right shoulder. In particular, the occurrence of such jitter becomes an important issue in the optical waveguide element having a transmission speed of a gigahertz order.
As a method of flattening the response characteristics of the electric signal applied to the optical waveguide element and the optical wave output from the optical waveguide element, there is known a method of applying the modulation signal of the driver 106 to the modulation electrode 103 of the optical waveguide element via the relay substrate 107 such as the filter circuit, or a method of connecting the termination circuit 108 such as a terminal resistor to a termination portion of the modulation electrode 103 as shown in FIG. 3. The optical waveguide element of FIG. 3 is obtained by forming the optical waveguide 102 on the substrate 101 made of a material having an electro-optic effect and the like and forming the modulation electrode 103 for modulating the optical wave propagating through the optical waveguide 102 (while the modulation electrode includes a signal electrode and a ground electrode, only an arrangement of the signal electrode is illustrated in FIG. 3 for the purpose of brevity). In addition, the input optical fiber 104 or the output optical fiber 105 is connected to the optical waveguide element, which is responsible for incidence of the optical wave to the optical waveguide element and emission of the optical wave from the optical waveguide element.
The optical waveguide element is housed in a single casing 109 along with a relay substrate 107, a termination circuit 108, or the like.
As a technique of using the termination circuit, there is known a method of improving the frequency characteristic of the optical modulator by adjusting the impedance of the termination portion of the modulation electrode of the optical modulator as disclosed in JP-B-3088988.
However, it is difficult to flatten the frequency characteristic up to the high frequency range that allows for several tens of Gbps transmission only using the termination circuit. It is also difficult to change the frequency to be adjusted out of the frequency characteristic of the electric/optical conversion response of the traveling-wave type optical modulator only by adjusting the impedance of the termination portion disclosed in JP-B-3088988.
A technique of using the relay substrate having a filter circuit is disclosed in JP-A-2007-10942 or JP-A-2008-83449. In order to flatten the frequency characteristic of the electric/optical response, a high pass filter obtained by connecting a capacitor 110 and a resistor 111 in parallel as shown in FIG. 4 is used as a basic configuration of the filter circuit. The reference numerals 112 and 113 of FIG. 4 denote electric lines of the relay substrate, and the reference numeral 107 denotes the relay substrate including the filter circuit.
In particular, JP-A-2007-10942 discloses a technique in which the capacitor or resistor of the filter circuit on the relay substrate is configured by a plurality of thin films on the electric line.
While a circuit configuration using such a thin film contributes to the miniaturization of the filter circuit, the manufacturing process becomes complicated, and in particular, a number of processes for forming and removing thin film are necessary to configure the capacitor using the thin film. In addition, although it is necessary to adjust the value of the capacitor or the resistor depending on the frequency characteristic of the optical waveguide element, it is possible to easily adjust the value of the resistor through simply trimming a part of thin film. However, it is difficult to adjust the value of the capacitor since the trimming may lead to a short circuit between the electrodes.
The inventors tried to address this problem for the capacitor by using a laminated ceramic capacitor.
Specifically, as shown in FIG. 5, the electric lines 112 and 113 are formed on the relay substrate main body 114, and a chip-type laminated ceramic capacitor 110 is arranged to connect two electric lines. The laminated ceramic capacitor is advantageous in that a capacitor having an extremely small size such as 0.3 mm vertical×0.6 mm horizontal×0.3 mm height (0603 size) can be obtained with low cost.
The laminated ceramic capacitor is formed so that the electrodes 122 and 123 have a comb tooth-shape so as to interpose the ceramic material 124 therebetween as shown in FIG. 6 (illustrating a cross-sectional view of the capacitor), and each electrode is connected to terminals 120 and 121 at both ends.
However, if the laminated ceramic capacitor is used, it was found that the frequency characteristic 130 of the optical response generates the resonance phenomenon (at a resonance frequency f0) as shown in FIG. 7. For this reason, it was considered that the laminated ceramic capacitor is unsuitable for a component of the filter circuit for connecting the optical waveguide element.
Moreover, the inventors also examined a technique of using a single-plate capacitor as the capacitor.
Specifically, as shown in FIG. 8, the electric lines 212 and 213 are formed on the relay substrate main body 214, and the single-plate capacitor 210 is arranged on one of the electric lines 212. In order to arrange the single-plate capacitor in the electric line, a conductive wire 220 such as a gold line is connected such that the electrode of the lower surface of the single-plate capacitor is electrically connected to the electric line 212, and the electrode of the upper surface of the single-plate capacitor is electrically connected to the electric line 213.
In the single-plate capacitor, as shown in FIG. 9 (illustrating a perspective view of a capacitor), the electrodes 215 and 217 are formed so as to interpose a dielectric material 216 therebetween. A relative dielectric constant ∈r of the dielectric material, a dielectric constant ∈0 of vacuum, the area S of the electrodes 215 and 217, a distance d of the electrodes 215 and 217, and an electrostatic capacitance C are expressed by the following formula.electrostatic capacitance C=∈r·∈0·S/d 
In general, since the single-plate capacitor has a high frequency characteristic better than that of the laminated ceramic capacitor, the single-plate capacitor is commonly used in devices where a high frequency characteristic is important.
However, since the thickness (distance d of electrodes) is changed in order to change the electrostatic capacitance of the single-plate capacitor having the same size, it is necessary to be careful in regard to the change of the characteristics caused by the thickness of the capacitor.
In particular, in the filter circuit described above, it was found that the resonance phenomenon (resonance frequency f0) generated in the frequency characteristic 130 of the electric/optical response shown in FIG. 7 is generated at a specific frequency range as the thickness of the capacitor becomes thicker, similar to the laminated ceramic capacitor, and a high frequency characteristic is deteriorated.